Thin Film Deposition
•  Physical processes
–  Evaporation: Thermal, E-beam, Laser, Ion-plating.
–  Sputtering: DC, RF, Magnetron, Reactive.
–  Spray: Flame, Plasma.
•  Chemical processes
–  Chemical Vapor Deposition (CVD): Thermal, MOCVD,
PECVD.
–  Plating: Electroplating, Electroless.
–  Solgel
–  ALE
•  Molecular Beam Epitaxy
Vacuum Evaporation
•  Heating a source material under vacuum until it
evaporates or sublimates.
•  The evaporant is deposited onto a substrate to
form a film.
•  Physical vapor deposition process.
•  High deposition rate, simple, easy to use.
•  Conductor materials in electronic circuits and
devices, dielectric and optical coatings.
Vacuum evaporation
Mean free path of atoms in the vapor > the distance from the source to the substrate.
The mean free path, λmfp, of particles in air at T = 25 oC (torr)
(cm)
n = P/kT
For N2 molecules d = 6.2 Å
(cm)
Vacuum evaporation
The flux of incident molecules is given by the HertzKnudsen equation
Flux , F = P / (2πmkT)1/2 [ molecules m-2 s-1 ]
where
P - gas pressure [ N m-2 ]
m - mass of one molecule [ kg ]
T - temperature [ K ]
F = nv/4,
n = P/kT, v = (8kT/πm)1/2
Vacuum evaporation
•  Evaporation coefficient: Larger for clean
surfaces, smaller for contaminated surfaces.
•  Deposition rate: Greatly depends on the
substrate-to-source geometry.
•  Non-uniform thickness on surface: Due to the
distance dependence. Knudsen’s cosine law,
cosθ/r2.
Thermal Evaporation
–  Difficulties: High melting point materials, uniformly
heating, rapidly change of deposition rate, reactions
between the source and the heating container.
E-beam evaporation
•  High energy focused electron
beam to heat the source material
at a small area.
•  Larger deposition rate.
•  Water-cooled container (cavity or
hearth): No source-container
reaction.
•  Sweeping or oscillating the ebeam to heat the source material
uniformly.
•  Multiple hearth sources: Different
source materials.
Vapor pressure
Ag
Phase diagram of AuSi
Vacuum evaporation
•  Low-energy process: the deposited material
condenses onto the substrate with very little
kinetic energy.
•  Kinetic energy ~ 0.5 eV.
•  Sputter deposition: Kinetic energy > 10 eV.
Sputter deposition
•  PVD process.
•  Bombardment and removal of the cathode
material with positive ions from a rare gas
discharge.
•  Originally developed to deposit refractory metals.
•  Now the sputter deposition is able to deposit
most materials.
Sputter deposition system
(1)
(3)
(4)
(5)
(6)
(2)
1) vacuum chamber 2) pumping system
3) sputter sources
4) substrate Anode
5) Heater
6) Gas flow supply
Sputtering Deposition Process
•  Sputtering
–  Ions are accelerated into target
–  Some of the surface atoms are sputtered off of the target.
–  These sputtered atoms “flow” across the chamber to where
they are deposited
http://www.utdallas.edu/~goeckner/plasma_sci_class/Plasma%20Process
%203%20Types.pdf
Mechanism of Sputtering
Sputtering involves the collisions of energetic ions with a target
surface and the process usually leads to ejection of target atoms.
Sputtering process is characterized by sputter yield, S, which is typically
in the range of 0.01 and 4 and increase with the mass of metals and
energy of the sputtering gas.
Sputter Yield
The sputter yield depends on: (a) the energy of the incident ions;
(b) the masses of the ions and target atoms; (c) the binding
energy of atoms in the solid and (d) the incident angle of ions.
60o – 70o
http://www.postech.ac.kr/mse/tfxs/2003_2/chapter3.pdf
Sputtering Alloy Targets
composition of alloy in film is approximately the same as alloy
in target (unlike evaporation)
slow diffusion mixing in
solids (sputtering)
• target reaches steady
state
• surface composition
balances sputter yield
http://www.postech.ac.kr/mse/tfxs/2003_2/chapter3.pdf
Advantages of sputter deposition
•  Elements, alloys and compounds can be sputtered and
deposited.
•  The sputtering target provides a stable, long-lived
vaporization source.
•  In some configurations, reactive deposition can be easily
accomplished using reactive gaseous species that are
activated in plasma.
•  The source and substrate can be spaced close together.
•  The sputter deposition chamber can have a small volume.
http://www.pfonline.com/articles/069901.html
Disadvantages of Sputter Deposition
•  Sputtering rates are low compared to those that can be
attained in thermal evaporation.
•  Sputtering targets are often expensive.
•  Most of the energy incident on the target becomes heat, which
must be removed.
•  In reactive sputter deposition, the gas composition must be
carefully controlled to prevent poisoning the sputtering
target.
http://www.pfonline.com/articles/069901.html
Basic Techniques
•
•
•
•
DC (diode) sputtering
RF (radio frequency) sputtering
Magnetron sputtering
Reactive sputtering
DC Sputtering
The simplest sputtering technology
E (e-) < 2eV - no ionization, elastic collisions only
E (e-) > 2eV - inelastic collisions add energy to Ar
Ø  ionization (highest energy process, ~15eV)
Note: mass (e-)/mass( Ar) ~ 10-5
•  energy transfer small
•  e- gain energy via elastic collisions until
E>15eV for ionization
•  #ions ~ #neutrals ~ 3 x 109 cm-3 @ 10mT
http://www.glue.umd.edu/~ddev/me489f/slides/2b_deposition_x6.pdf
DC Sputtering (Cont.)
Light e- pulled towards walls faster
than ions, leaving slightly more
ions in glow region
Light e- move away from cathode
faster than ions, leading to a large
field, high acceleration of ions into
cathode
high-E ions (10keV to 1 MeV)
knock target material loose
resulting plume of neutrals
new electrons from impact
reactions replenish the plasma
http://www.glue.umd.edu/~ddev/me489f/slides/2b_deposition_x6.pdf
Operating Pressure for DC sputtering
Operating pressure limitations are
imposed by the requirement of both
the glow discharge and of film
deposition.
Optimum deposition
rate around 100 mTorr
Milton Ohring, Materis Science of
Thin Film, second Edition, P208
Parameters for DC Sputtering
•  Sputter voltage
–  typically -2 to -5 kV
•  Substrate Bias Voltage
–  substrate is being bombarded by electrons and ions from target and
plasma
•  sputtering film while you deposit
–  neutral atoms deposit independently
–  put negative bias on the substrate to control this
–  can significantly change film properties
•  Deposition rate
–  changes with Ar pressure
–  increases with sputter yield
•  usually increases with high voltage
http://www.uccs.edu/~tchriste/courses/PHYS549/549lectures/
sputtertech.html
RF Sputtering
•  DC sputtering - what about dielectrics?
•  in DC systems, positive charge builds up on the cathode
(target) need 1012 volts to sputter insulators !!
avoid charge build up by alternating potential
RF sputtering
http://www.uccs.edu/~tchriste/courses/PHYS549/549lectures/
sputtertech.html
RF Sputtering
•  frequencies less than about 50 kHz
–  electrons and ions in plasma are mobile
•  both follow the switching of the anode and cathode
–  basically DC sputtering of both surfaces
•  frequencies above about 50 kHz
–  ions (heavy) can no longer follow the switching
–  enough electrons to ionize gases (5~30MHz)
Typically 13.56 MHz is used
Advantages of RF Sputtering
•  It works well with insulating targets
•  High efficiency
easier to keep plasma going → can operate at lower
Ar pressures (1-15 mTorr) → fewer gas collisions
→ more line of sight deposition
http://aultimut.com/aultimut/details.asp?itemid=11
Magnetron Sputter Deposition
v Use with DC or RF
•  High sputtering efficiency
increase ionization of Ar
–  Why? Higher sputter rates at lower Ar pressures
(down to 0.5 mTorr)
fewer gas collisions - more line of sight
–  How ? increase probability of electrons striking Ar
–  increase electron path length
–  use electric and magnetic fields
Magnetron Sputtering Principle
This technology uses powerful magnets to confine the “glow discharge”
plasma to the region closest to the target plate. That vastly improves the
deposition rate by maintaining a higher density of ions, which makes the
electron/gas molecule collision process much more efficient.
http://www.angstromsciences.com/technology/sputtering.htm
Advantages of Magnetron Sputtering
•  High deposition rate
•  Reducing electron bombardment of substrate
•  Extending the operating vacuum range
–  ability to operate at lower pressures
The most widely commercially practiced sputtering method
Parameters for Magnetron Sputtering
•  Deposition pressure : 10-3 to 0.1 Pa ( 10-5 to 10-3 torr)
•  Deposition rate : 0.2 ~ 2-6 m/min (10 times higher than
conventional sputtering)
•  Deposition temperature : 100 to 150 oC
http://engineering.dartmouth.edu/other/microeng/courses/es194/student/jiaying/sem/II.
2.p4.html
Disadvantages for Magnetron Sputtering
•  An erosion track in the target
–  This leads to poor efficiency of sputtering yield versus
target volume compared to non-magnetron sputtering
•  Non-uniform removal of particles from target result in
non-uniform films on substrate
http://www.mse.ncsu.edu/WideBandgaps/classes/MAT%20751%20S03/Sputtering/
Magnetron_Ihlefeld.pdf
Reactive Sputtering
Sputtering metallic target in the presence
of a reactive gas mixed with inert gas (Ar)
• A mixture of inert +reactive gases used for sputtering
oxides – Al2O3, SiO2, Ta2O5 (O2)
nitrides – TaN, TiN, Si3N4 (N2, NH3)
carbides – TiC, WC, SiC (CH4, C2H4, C3H8)
Reactive Sputtering (Cont.)
•  chemical reaction takes place on substrate and target
•  can poison target if chemical reactions are faster than
sputter rate
•  adjust reactive gas flow to get good stoichiometry
without incorporating excess gas into film
Reactive Magnetron Sputtering
Zinc Oxide Thin films
•
Zinc oxide is one of the most interesting II/IV compound semiconductors It has
been investigated extensively because of its interesting electrical, optical and
piezoelectric properties
Reactive sputtering
is the best technique
for Zinc Oxide
deposition.
R. Ondo-Ndong , F. Pascal-Delannoy, A.
Boyer, A. Giani, A. Foucaran, Materials
Science and Engineering B97 (2003) 68 /73
Process Parameters
Quality of the film dependents on deposition conditions, such as
substrate temperature, deposition power, deposition pressure
and argon–oxygen flow.
Walter Water, Sheng-Yuan Chu*, Materials Letters 55 (2002) 67– 72
Energy sources and Reactor types
Thermal Energy
resistive heating - tube furnace
quartz tungsten halogen lamp (very good heat source) - radiant heating
radio-frequency - inductive heating
laser as thermal energy source
Photo Energy
UV-visible light
laser as photo energy source
Laser ablation
•
•
•
•
•
Pulsed laser deposition (PLD).
Ultraviolet (UV) light (~200 nm – 400 nm).
High energy (> 1 J/cm2).
Pulsed, not continuous, laser beam.
Excimer laser: Gas laser, F2 (157 nm), ArF (193
nm), KrCl (222 nm), KrF (248 nm), XeCl (308
nm), XeF (351 nm).
•  Nd3+ :YAG laser: Solid state laser, Nd ions,
yttrium aluminum garnet (YAG), 1064 nm,
frequency doubled to 532 nm, mixed with 1064
nm to produce 355 nm or 266 nm.
PLD system
•  Lasers: Excimer laser. 1 to 100 Hz repetition
rates, 15 to 50 ns pulse duration.
•  Optics: UV windows, mirrors, beam splitters.
•  Vacuum chamber, pumps and gauges: High
vacuum standard equipments.
•  Gas flow: Oxygen.
•  Target manipulation: Rotation.
•  Substrate holder and heater.
Chemical vapor deposition
•  Reactive carrier gases.
•  Transport precursors of desired materials to the
substrate surface.
•  Gases react with other gases or decompose to
produce stable reaction products and deposit on
the substrate.
•  Most versatile deposition techniques.
•  Wide range of chemical reactions, reactants, and
applications.
Films grown by CVD
•
•
•
•
Single-crystalline thin films.
Polycrystalline thin films.
Amorphous thin films.
Semiconductors (IV, III-V, II-VI), metals,
dielectric, superconductors.
•  Microelectronics, optoelectronics, hard and
protective coatings.
Chemical vapor deposition (CVD) : overview
•  CVD (thermal)
–  APCVD (atmospheric)
–  LPCVD (<10 Pa)
–  VLPCVD (<1.3 Pa)
•  PE CVD (plasma enhanced)
•  Photon-assisted CVD
•  Laser-assisted CVD
•  MOCVD
Tensile stress causes concave
bending of a thin substrate
Deposited film
Compressive stress causes convex
bending of a thin substate
Deposited film
•
•
•
•
•
•
•
•
•
Chemical vapor deposition: reaction mechanisms
Diffusive-convective transport of
Mass transport of the reactant in
CVD:depositing
species to a substrate
the bulk
Gas-phase reactions
(homogeneous)
Mass transport to the surface
Adsorption on the surface
Surface reactions
(heterogeneous)
Surface migration
Incorporation of film
constituents, island formation
Desorption of by-products
Mass transport of by-produccts
in bulk
with many intermolecular
collisions-driven by a concentration
gradient
SiH
SiH4
4
Si
Chemical Vapor Deposition
Precursor
Reactor
Solid Products
(thin films and powders)
Gas Phase products
Energy
Precursor Considerations
Volatility
vapor pressure - simple molecules with high vapor pressure are rare
determined by molecular weight and molecularity (degree polymerized) - result of structure and bonding
control - temperature, valving
Stability, Reactivity, and Safety
bond strength, bond dissociation energy - affects process temperature and film composition (purity)
thermal stability in storage and delivery into the reactor
reactivity of the precursor and byproducts towards other substances (including biological objects like us)
Single-Source Precursor
providing more than one element into the film
simpler delivery system
uniform elemental distribution at atomic level possible
limited composition range
Common Precursors
hydrides: MHx SiH4, GeH4, AlH3(NMe3)2, NH3, PH3 ...
halides: MXy TiCl4, TaCl5, MoF6, WF6, ...
metal-organics metal alkyls: AlMe3, AliBu3, Ti(CH2tBu)4 ....
metal alkoxides: Ti(OiPr)4, [Cu(OtBu)]4 ....
metal dialkylamides: Ti(NMe2)4, Cr(NEt2)4 ....
metal diketonates: Cu(acac)2, Pt(hfac)2 ....
metal carbonyls: Fe(CO)5, Ni(CO)4 ....
others: complexes with alkene, allyl, cyclopentadienyl, ..... ligands
many precursors have mixed ligands
http://chiuserv.ac.nctu.edu.tw/~htchiu/cvd/essential.html
Thermally activated CVD
•  Thermal energy (resistance heating, RFheating, infrared radiation).
•  Normal or low pressure.
•  Inorganic materials.
Plasma-enhanced CVD
•
•
•
•
•
Use plasma to ionize and dissociate gases.
Enhance growth rate.
Lower temperatures.
Low pressure.
Deposite SiO2, Si3N4 (temperature<350oC).
PECVD
Photo-assisted CVD
•  Use light to enhance the reaction rate.
•  The effect of radiation is either a local heating
of the water or a photochemical reaction.
•  Photochemical reaction: Photolytic reaction.
Laser Assisted Vapor Deposition
He
H2
SiH4, SF6, H2
Chemical vapor deposition (CVD) : L-CVD
•
The L-CVD method is able to fabricate
continuous thin rods and fibres by pulling the
substrate away from the stationary laser focus
at the linear growth speed of the material
while keeping the laser focus on the rod tip, as
shown in the Figure . LCVD was first
demonstrated for carbon and silicon rods.
However, fibers were grown from other
substrates including silicon, carbon, boron,
oxides, nitrides, carbides, borides, and metals
such as aluminium. The L-CVD process can
operate at low and high chamber pressures.
The growth rate is normally less than 100 µm
/s at low chamber pressure (<<1 bar). At high
chamber pressure (>1 bar), high growth rate
(>1.1 mm/s) has been achieved for small
-diameter (< 20 µm) amorphous boron fibers.
Laser Assisted Catalytic growth
Fig. 2 Schematic of a nanowire growth set up using a combination of laser ablation and
vapor-liquid- solid scheme
Fig.1 Semiconductor (GaAs) nanowires grown using laser
assisted catalytic growth [1ref]. The scale bar corresponds to
50 nm.
MOCVD
•
•
•
•
Thermal, glow discharge, ultraviolet radiation.
Metalorganic compound (MO) gases or liquids.
Epitaxial growth.
Semiconductors (III-V, II-VI), high Tc
superconductors, dielectric and metal films.
MOCVD
H2+AsH3
RF Coils
Gas
Al(CH3)3
Ga(CH3)3
H2
Exhaust
Mixing Chamber
Substrate
Ga(CH3)3 + AsH3 => 3CH4+GaAs
Comparison of CVD
•
•
•
•
•
Thermal CVD: Thermal, inorganic sources.
MOCVD: Thermal, organometallic sources.
PECVD: Plasma, low temperatures.
PACVD: Light, low temperatures, selected area.
ALE: Thermal/light, monolayer control.
Pressure ranges
•
•
•
•
•
TACVD: 10-5 – 760 Torr.
MOCVD: 10 – 760 Torr.
PECVD: 0.01 – 10 Torr.
PACVD: 10 – 760 Torr.
ALE: 0.01 – 760 Torr.
CVD deposition system
•  Rough vacuum chamber and sensors.
•  Source materials (gas, vapor), gas carriers, gas
flow control system.
•  Chemical reaction sources: thermal heating
(heaters), plasma generating sources to provide
the needed energy for the chemical reactions.
•  Substrates, holders.
Growth Mechanism
•  Gas-phase: homogeneous reaction, diffusion
of reactants to the substrate surface (mass
transport), desorption of reaction by-products
from the surface, diffusion of products into the
main gas stream.
•  Surface: adsorption of reactants at surface,
heterogeneous chemical reactions, surface
migration, lattice incorporation.
Reduction
•  Many deposition reactions in CVD.
•  Decomposition reaction favoured by the
presence of another reactant (usually H2)
which helps to remove one or several of
the decomposition products.
•  The reversed reaction leads to etching.
•  Reversible reaction.
•  High temperatures.
Oxidation
•  The reaction of a vapor-phas substance with
oxygen or oxidant (CO2, N2O, NO, O3), resulting
in a solid film (oxide).
•  Amorphous dielectric films (SiO2, Al2O3, TiO2),
silicates glasses, oxide semiconductors (ZnO),
oxide conductors (SnO2, In2O3), magnetic films
(LCMO).
Hydrolysis
•  The gaseous compound reacts with externally
or in situ formed water vapour leading to the
formation of a solid film, usually an oxide.
•  Amorphous and epitaxial films.
Nitridation
•  The reaction between a gaseous reactant and
ammonia (NH3), nitrogen, hydrazine (N2H4) or
another nitrogen-containing compound, leading
to the formation of a thin nitride film.
•  Silicon nitride (Si3N4), oxynitrides (SixOyNz,
AlxOyNz), semiconducting compound nitrides
(BN, AlN, GaN), metallic nitrides (TaN, TiN) and
superconducting nitrides (NbN).
Chemical Transport
•  The transfer of a relatively non-volatile solid
source reacting with a transport agent to form a
volatile species.
Disproportion
•  The dissociation of a reactant species due to its
instability at a lower temperature and the
formation of another more stable higher-valence
chemical species,yielding at the same time the
elemental form of the reactant to be deposited.
•  A typical reaction: GaCl -> (low temp.) Ga +
GaCl3, GaCl<- (high temp.) Ga + GaCl3.
Other reactions in CVD
•  Catalysis: enhanced velocity of a reaction.
•  Synthesis: a solid film results due to the reaction.
•  Photolysis: a reaction due to the absorption of
an ultraviolet light.
•  Combined reactions: more than one reactions in
a reaction.
Atomic Layer Deposition
•  Surface controlled for epitaxial growth of single
crystal.
•  Originally designed for compound
semiconductors, II-VI, III-V, oxides, nitrides.
•  Epitaxy: Sequential saturated surface reactions
to form a monolayer in each sequence.
•  Release or exchange of ligands for saturation in
each sequence.
ALD Process and Equipments
•  Releases sequential precursor gas pulses to deposit a
film one layer at a time.
•  A first precursor gas is introduced into the process
chamber and produces a monolayer of gas on the
wafer surface. Then a second precursor of gas is
introduced into the chamber reacting with the first
precursor to produce a monolayer of film on the
wafer surface.
Two fundamental mechanisms:
§  Chemisorption saturation process
§  Sequential surface chemical reaction process
Ref: "Atomic Layer Deposition," Cambridge NanoTech Inc., 24
April 06. <http://www.cambridgenanotech.com/>.
Example: ALD cycle for Al2O3 deposition
ALD Process and Equipments
•
•
Releases sequential precursor gas pulses to deposit a film one layer at a time.
A first precursor gas is introduced into the process chamber and produces a
monolayer of gas on the wafer surface. Then a second precursor of gas is
introduced into the chamber reacting with the first precursor to produce a
monolayer of film on the wafer surface.
Two fundamental mechanisms:
§  Chemisorption saturation process
§  Sequential surface chemical reaction process
•  Example: ALD cycle for Al2O3 deposition (after 3 cycles)
Ref: "Atomic Layer Deposition," Cambridge NanoTech Inc.,
24 April 06. <http://www.cambridgenanotech.com/>.
•  Advantages
§  Stoichiometric films with large area uniformity and 3D
conformality.
§  Precise thickness control.
§  Low temperature deposition possible.
§  Gentle deposition process for sensitive substrates.
•  Disadvantages
§  Deposition Rate slower than CVD.
§  Number of different material that can be deposited is fair
compared to MBE.
Plating
•  Deposition of a metal film using chemical reaction.
•  Two types: Electroplating, electroless plating.
•  Electroplating: Electrical contact required,
conductive substrates, um thick films.
•  Electroless plating: No electrical contact required,
dielectrics and conductors, thin films.
Plating
•
•
•
•
Non-vacuum system.
Electroplating
Electroless Plating
Applications
–  Anticorrosion coatings, interconnect
metallization formation.
–  Au, Ni coating for wire bonding.
–  Device interconnections.
Molecular beam epitaxy
•  Epitaxy: Continuation of crystal structure from
the substrate to the film.
•  Single crystalline contaminant-free film.
•  Single atomic layer (monolayer) growth.
•  Small mismatched lattice constant (<1%).
•  Ultrahigh vacuum: <5x10-11 torr.
•  Non-epitaxy films: amorphous or polycrystalline.
Molecular Beam Epitaxy
MBE deposition system
•
•
•
•
Load lock for substrate loading & unloading.
Liquid nitrogen shroud to cool the wall.
Substrate heating: clean surface.
Film growth: ultrahigh vacuum, shutters to
control the flux, in-situ analysis and thickness
monitor (RHEED) to control the deposition rate.
RHEED
•
•
•
•
•
Reflection high energy electron diffraction.
High energy: > 10 KeV.
Small (grazing) angle incidence (<3°).
Low depth penetration: < 100 A.
Lateral ordered structure: Surface structure
symmetry, lattice constant.
RHEED
MBE film growth
•  Single crystalline substrate with lattice matching
to the film.
•  Heating (500oC to 550oC for GaAs, 700oC to
900oC for Si) to remove the surface
contamination.
•  Layer by layer growth: controlled by high speed
shutters and thickness monitor (or RHEED).
Quantum Dots Grown Using MBE
“Electronic Structure of InAs Pyramidal Quantum Dots”:
http://www.sst.nrel.gov/research/InAs.html
AFM
Three-dimensional AFM image of CdSe QDs deposited on
ZnCdMgSe barriers. The inset shows a histogram of the
QD height [ Courtesy: Prof. Tamargo’S group- CCNY ].